FCM literally means "Five Cock Minimum", it is used to describe something that is incredibly homosexual. To FCM someone is to, while they are sleeping, place either a used condom or an unrolled condom that is full or skeet or lotion and the place upon the sleepers face. Then take copius amounts of pictures and blackmail that person.

The word orginiated in Miami FL, when a certain individual brought condom and porn with him on a school trip and decided to leave them on top of another certain individual's luggage. As soon as we saw dat shit we was like "Aaawwwww Hell naw, we aint takin this shit" So den um proceeded to fillin the rubber with some of dat free lotion you usually end up stealin, an' we den got dat porn he was frontin wit and open it to a page where this ho wuz takin lak 5 guys at once so we put da picture on his lap and took mad crazy pictures and when we tol him lak a monf afta he was all like "how could i let this happen" and wuz crying it was mad funny.

FCM stands for "Friendly Clan Match". It basically means two apposing clans (Gamer clans, such as in Counter Strike) combat each other, not for official purposes, but rather for good old fasioned fun or paractice.